Hermetic compressors, including those of the scroll type, are of a high or a low side type. A high side compressor is one in which the motor is disposed in the discharge or high pressure portion of the compressor shell. A low side compressor is one in which the motor is disposed in the suction or low pressure portion of the hermetic shell.
A common problem in hermetic rotary compressors, including those of the scroll type, is the tendency of compressed refrigerant gas to flow back from the discharge pressure portion of the compressor shell, through the compression mechanism and back to the suction side of the shell upon compressor shutdown. This backflow is as a result of the natural tendency of the system within which the compressor is employed to equalize its internal pressure when the compressor is de-energized. Such backflow, if not prevented, can cause the high speed reverse rotation of the compression mechanism and can lead to potentially serious compressor damage.
The prevention of such backflow upon compressor shutdown is typically accomplished by the disposition of a discharge check valve downstream of the aperture through which gas is discharged from the compressor's compression mechanism. The discharge check valve is closed by the initial backflow of refrigerant gas through the compressor which begins immediately upon compressor shutdown. The closing of the discharge check valve may be assisted or accelerated by a biasing member such as a spring.
In scroll compressors having compression mechanisms which are protected from gas-driven reverse rotation by apparatus such as a discharge check valve, a problem arises when the compressor is electrically connected in an improper manner. Such improper electrical connection can cause the motor to run in a direction reverse from which it is intended to run. This problem is recognized in U.S. Pat. Nos. 4,820,130 and 4,840,545, both of which are assigned to the assignee of the present invention.
Briefly, when a scroll compressor having a discharge check valve is miswired so that it is caused to run backwards, the pockets defined between the scroll wraps, instead of moving radially inward and decreasing in volume, move radially outward and expand in volume in a pumping action. In effect, the scroll device functions as a gas expander or pump as opposed to a compressor.
The expansion of the pockets defined by the scroll members under such circumstances causes low and even negative pressures to develop within the pockets because the discharge check valve, being closed, gives the mechanism no source of gas to pump from. As a result, the scroll members are drawn tightly together which can eventually result, to the extent the compressor motor continues to run backwards, in severe damage and possibly destruction of the compressor.
Still another difficulty and potential source for damage in scroll compressors is the development of high discharge gas temperatures in operation. Such high discharge temperatures can result from, among other things, the operation of the compressor in a system where pressure ratios develop that are outside of the compressor's normal operating range. Such high discharge gas temperatures can cause thermal growth within the compressor, and, in particular, thermal growth of the scroll wraps. The thermal expansion of the scroll wraps can lead to high wrap tip contact loads and the galling of the wrap tips.
Compressor protection with respect to the development of high discharge temperatures has historically involved the disposition of a temperature sensor on a discharge line leading from the compressor's hermetic shell or the disposition of an internally mounted temperature sensor closely proximate to the location at which discharge gas issues from between the scroll wraps into the discharge portion of the compressor shell. The former arrangement can be inadequate because the externally mounted sensor, which is remote from the critical scroll wrap location, may not sense the existence of high discharge temperatures sufficiently early to prevent damage to the scroll members.
The latter arrangement, employing an internally mounted temperature sensor, while faster acting than arrangements employing externally mounted sensors, requires the mounting of the sensor in the discharge pressure portion of the compressor's hermetic shell. As a result, in low side compressors the leads of a sensor mounted in the discharge pressure portion of the shell must be routed out of the hermetic shell or at least out of the discharge pressure portion of the shell in order for the signal produced by the sensor to be used to shut down the compressor's motor.
The need continues to exist to protect hermetic scroll compressors of the low side type from the damage which can result from their improper electrical hookup or from the occurrence of high discharge temperatures, while eliminating the need to position a temperature sensor in the discharge portion of the compressor shell and the need to route sensor leads through or out of the shell's discharge pressure portion.